Email swallow somewhere in Comcast - how to get support

I've managed to convince most people I communicate with that have a comcast email account to use something else, like gmail.  Unfortunately one person I communicate with has not done so.  October 30 is the last time I can confirm that I replied to an email he sent (he sent to multiple recipients) and I replied to them all and where everyone else got the email.  He replied a few days later to say he'd take action when he heard from me.  Since then a handful of emails have been exchanged where he is not seeing my email, only reply-all responses to my email sent by other recipients.

This person is a non-technical user.

Who at Comcast can I contact to look into this.